PKA (irradiation)
A Primary Knock-on Atom or PKA is an atom that is displaced from its lattice site by irradiation; it is, by definition, the first atom that an incident particle encounters in the target. After it is displaced from its initial lattice site, the PKA can induce the subsequent lattice site displacements of other atoms if it possesses sufficient energy, or come to rest in the lattice at an interstitial site if it does not.
Most of the displaced atoms resulting from electron irradiation and some other types of irradiation are PKAs, since these are usually below the threshold displacement energy and do not have sufficient energy to displace more atoms. In other cases like fast neutron irradiation, most of the displacements result from higher energy PKAs colliding with other atoms as they slow down to rest.
Atoms can only be displaced if, upon bombardment, the energy they receive exceeds a threshold energy Ed. Likewise, when a moving atom collides with a stationary atom, both atoms will have energy greater than Ed after the collision only if the original moving atom had an energy exceeding 2Ed. Thus, only PKAs with an energy greater than 2Ed can continue to displace more atoms and increase the total number of displaced atoms. In cases where the PKA does have sufficient energy to displace further atoms, the same truth holds for any subsequently displaced atom.
In any scenario, the majority of displaced atoms leave their lattice sites with energies no more than two or three times Ed. Such an atom will collide with another atom approximately every mean interatomic distance traveled, losing half of its energy during the average collision. Assuming that an atom that has slowed down to a kinetic energy of 1 eV becomes trapped in an interstitial site, displaced atoms will typically be trapped no more than a few interatomic distances away from the vacancies they leave behind.
There are several possible scenarios for the energy of PKAs, and these lead to different forms of damage. In the case of electron or gamma ray bombardment, the PKA usually does not have sufficient energy to displace more atoms. The resulting damage consists of a random distribution of Frenkel defects, usually with a distance no more than four or five interatomic distances between the interstitial and vacancy. When PKAs receive energy greater than Ed from bombarding electrons, they are able to displace more atoms, and some of the Frenkel defects become groups of interstitial atoms with corresponding vacancies, within a few interatomic distances of each other. In the case of bombardment by fast-moving atoms or ions, groups of vacancies and interstitial atoms widely separated along the track of the atom or ion are produced. As the atom slows down, the cross section for producing PKAs increases, resulting in groups of vacancies and interstitials concentrated at the end of the track.
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